$z-layers: (
  "submit-box": 10,
  "write-header": 15,
  "modal": 100,
  "modal-content": 110,
  "upload-mask": 120
);

@function z($layer) {
  @if not map-has-key($z-layers, $layer) {
    @warn "No layer found for `#{$layer}` in $z-layers map. Property omitted.";
  }

  @return map-get($z-layers, $layer);
}
